In vehicles, security-relevant functions are increasingly based on electrically operated components. It is therefore essential to ensure that these components can be supplied at all times with sufficient electrical energy so that their functional capability is ensured. The generator and the battery are available as providers of the electrical energy in the vehicle, it being the task of the battery to compensate for short-term loads. For it to be capable of fulfilling this task, it is necessary to be able to make statements about its performance capability at any time. Therefore, there are already multiple suggestions as to how the performance capability of a battery, in particular a vehicle battery, may be ascertained continuously.
A measure of the performance capability of a battery is its internal resistance. A completely operational battery has a lower internal resistance than an aged battery, which is no longer very powerful. Because it is known that the internal resistance of the battery is a measure of its performance capability, there are already suggestions as to how the internal resistance of a battery may be ascertained and the functional capability of the battery may be inferred therefrom. A method and a device for determining the internal resistance of a battery are known, for example, from DE 102 08 020 A1. In this known method, the internal resistance of a motor vehicle battery is determined by calculating the quotient of a voltage change in relation to a current change in the event of a vehicle electrical system excitation. A vehicle electrical system excitation is understood, for example, as the connection or disconnection of a powerful electrical consumer, which causes voltage oscillations. In the known method, it is necessary to continuously measure both the flowing current and also the voltage. Because the quality of the determined internal resistance is very heavily dependent on the level of the vehicle electrical system excitation, supplementary methods are necessary, which determine the quality or significance of the particular instantaneously ascertained internal resistance via the amplitude or power of the vehicle electrical system excitation and take it into consideration in a downstream filter.
Because the internal resistance of a battery is ascertained as a function of current and voltage oscillations in the known method and significant difficulties may occur in this, it is suggested that multiple parallel filters having different time constants additionally be used for the current and voltage signal. The starting values are evaluated and finally used for releasing the instantaneously ascertained internal resistance. The necessary case differentiations and dependencies of the individually employed modules on one another make systematic analysis of the overall system and the overall functionality more difficult in all operating cases.